OAuth Working Group B. Campbell
Internet-Draft Ping Identity
Intended status: Standards Track J. Bradley
Expires: November 7, 2018 Yubico
N. Sakimura
Nomura Research Institute
T. Lodderstedt
YES Europe AG
May 6, 2018
OAuth 2.0 Mutual TLS Client Authentication and Certificate Bound Access
Tokens
draft-ietf-oauth-mtls-08
Abstract
This document describes OAuth client authentication and certificate
bound access tokens using mutual Transport Layer Security (TLS)
authentication with X.509 certificates. OAuth clients are provided a
mechanism for authentication to the authorization sever using mutual
TLS, based on either single certificates or public key infrastructure
(PKI). OAuth authorization servers are provided a mechanism for
binding access tokens to a client's mutual TLS certificate, and OAuth
protected resources are provided a method for ensuring that such an
access token presented to it was issued to the client presenting the
token.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 7, 2018.
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Internet-Draft OAuth Mutual TLS May 2018
Mutual TLS certificate bound access tokens and mutual TLS client
authentication are distinct mechanisms, which are complementary but
don't necessarily need to be deployed or used together.
1.1. Requirements Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
Mutual TLS refers to the process whereby a client presents its X.509
certificate and proves possession of the corresponding private key to
a server when negotiating a TLS session. In TLS 1.2 [RFC5246] this
requires the client to send Client Certificate and Certificate Verify
messages during the TLS handshake and for the server to verify these
messages.
2. Mutual TLS for OAuth Client Authentication
This section defines, as an extension of OAuth 2.0, Section 2.3
[RFC6749], two distinct methods of using mutual TLS X.509 client
certificates as client credentials. The requirement of mutual TLS
for client authentication is determined by the authorization server
based on policy or configuration for the given client (regardless of
whether the client was dynamically registered, statically configured,
or otherwise established).
In order to utilize TLS for OAuth client authentication, the TLS
connection between the client and the authorization server MUST have
been established or reestablished with mutual TLS X.509 certificate
authentication (i.e. the Client Certificate and Certificate Verify
messages are sent during the TLS Handshake [RFC5246]).
For all requests to the authorization server utilizing mutual TLS
client authentication, the client MUST include the "client_id"
parameter, described in OAuth 2.0, Section 2.2 [RFC6749]. The
presence of the "client_id" parameter enables the authorization
server to easily identify the client independently from the content
of the certificate. The authorization server can locate the client
configuration using the client identifier and check the certificate
presented in the TLS Handshake against the expected credentials for
that client. The authorization server MUST enforce the binding of a
certificate to a specific client as described in either Section 2.1
or Section 2.2 below.
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Internet-Draft OAuth Mutual TLS May 20182.1. PKI Mutual TLS OAuth Client Authentication Method
The PKI (public key infrastructure) method of mutual TLS OAuth client
authentication uses a subject distinguished name (DN) and validated
certificate chain to identify the client. The TLS handshake is
utilized to validate the client's possession of the private key
corresponding to the public key in the certificate and to validate
the corresponding certificate chain. The client is successfully
authenticated if the subject information in the certificate matches
the expected DN configured or registered for that particular client
(note that a predictable treatment of DN values, such as the
distinguishedNameMatch rule from [RFC4517], is needed in comparing
the certificate's subject DN to the client's registered DN). If and
how to check a certificate's revocation status is a deployment
decision at the discretion of the authorization server. The PKI
method facilitates the way X.509 certificates are traditionally being
used for authentication. It also allows the client to rotate its
X.509 certificates without the need to modify its respective
authentication data at the authorization server by obtaining a new
certificate with the same subject DN from a trusted certificate
authority (CA).
2.1.1. PKI Authentication Method Metadata Value
For the PKI method of mutual TLS client authentication, this
specification defines and registers the following authentication
method metadata value into the "OAuth Token Endpoint Authentication
Methods" registry [IANA.OAuth.Parameters].
tls_client_auth
Indicates that client authentication to the authorization server
will occur with mutual TLS utilizing the PKI method of associating
a certificate to a client.
2.1.2. Client Registration Metadata
The following metadata parameter is introduced for the OAuth 2.0
Dynamic Client Registration Protocol [RFC7591] in support of the PKI
method of binding a certificate to a client:
tls_client_auth_subject_dn
An [RFC4514] string representation of the expected subject
distinguished name of the certificate the OAuth client will use in
mutual TLS authentication.
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Internet-Draft OAuth Mutual TLS May 20182.2. Self-Signed Certificate Mutual TLS OAuth Client Authentication Method
This method of mutual TLS OAuth client authentication is intended to
support client authentication using self-signed certificates. As
pre-requisite, the client registers an X.509 certificate or a trusted
source for its X.509 certificates (such as the "jwks_uri" defined in
[RFC7591] that references a JSON Web Key [RFC7517] Set containing the
client's certificates and public keys) with the authorization server.
During authentication, TLS is utilized to validate the client's
possession of the private key corresponding to the public key
presented within the certificate in the respective TLS handshake. In
contrast to the PKI method, the client's certificate chain is not
validated by the server in this case. The client is successfully
authenticated if the subject public key info of the certificate
matches the subject public key info of one of the certificates
configured or registered for that particular client. The Self-Signed
Certificate method allows to use mutual TLS to authenticate clients
without the need to maintain a PKI. When used in conjunction with a
"jwks_uri" for the client, it also allows the client to rotate its
X.509 certificates without the need to change its respective
authentication data directly with the authorization server.
2.2.1. Self-Signed Certificate Authentication Method Metadata Value
For the Self-Signed Certificate method of mutual TLS client
authentication, this specification defines and registers the
following authentication method metadata value into the "OAuth Token
Endpoint Authentication Methods" registry [IANA.OAuth.Parameters].
self_signed_tls_client_auth
Indicates that client authentication to the authorization server
will occur using mutual TLS with the client utilizing a self-
signed certificate.
2.2.2. Client Registration Metadata
For the Self-Signed Certificate method of binding a certificate to a
client using mutual TLS client authentication, the existing
"jwks_uri" or "jwks" metadata parameters from [RFC7591] are used to
convey the client's certificates and public keys, where the X.509
certificates are represented using the JSON Web Key (JWK) [RFC7517]
"x5c" parameter (note that Sec 4.7 of RFC 7517 requires that the key
in the first certificate of the "x5c" parameter must match the public
key represented by other members of the JWK).
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Internet-Draft OAuth Mutual TLS May 20183. Mutual TLS Client Certificate Bound Access Tokens
When mutual TLS is used by the client on the connection to the token
endpoint, the authorization server is able to bind the issued access
token to the client certificate. Such a binding is accomplished by
associating the certificate with the token in a way that can be
accessed by the protected resource, such as embedding the certificate
hash in the issued access token directly, using the syntax described
in Section 3.1, or through token introspection as described in
Section 3.2. Binding the access token to the client certificate in
that fashion has the benefit of decoupling that binding from the
client's authentication with the authorization server, which enables
mutual TLS during protected resource access to serve purely as a
proof-of-possession mechanism. Other methods of associating a
certificate with an access token are possible, per agreement by the
authorization server and the protected resource, but are beyond the
scope of this specification.
The client makes protected resource requests as described in
[RFC6750], however, those requests MUST be made over a mutually
authenticated TLS connection using the same certificate that was used
for mutual TLS at the token endpoint.
The protected resource MUST obtain the client certificate used for
mutual TLS authentication and MUST verify that the certificate
matches the certificate associated with the access token. If they do
not match, the resource access attempt MUST be rejected with an error
per [RFC6750] using an HTTP 401 status code and the "invalid_token"
error code.
Metadata to convey server and client capabilities for mutual TLS
client certificate bound access tokens is defined in Section 3.3 and
Section 3.4 respectively.
3.1. X.509 Certificate Thumbprint Confirmation Method for JWT
When access tokens are represented as JSON Web Tokens (JWT)[RFC7519],
the certificate hash information SHOULD be represented using the
"x5t#S256" confirmation method member defined herein.
To represent the hash of a certificate in a JWT, this specification
defines the new JWT Confirmation Method [RFC7800] member "x5t#S256"
for the X.509 Certificate SHA-256 Thumbprint. The value of the
"x5t#S256" member is the SHA-256[SHS] hash (a.k.a. thumbprint,
fingerprint or digest) of the DER encoding of the X.509 certificate
[RFC5280] base64url-encoded [RFC4648] with with all trailing pad '='
characters omitted and without the inclusion of any line breaks,
whitespace, or other additional characters.
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The following is an example of a JWT payload containing an "x5t#S256"
certificate thumbprint confirmation method.
{
"iss": "https://server.example.com",
"sub": "ty.webb@example.com",
"exp": 1493726400,
"nbf": 1493722800,
"cnf":{
"x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
}
}
Figure 1: Example JWT Claims Set with an X.509 Certificate Thumbprint
Confirmation Method
If, in the future, certificate thumbprints need to be computed using
hash functions other than SHA-256, it is suggested that additional
related JWT confirmation methods members be defined for that purpose.
For example, a new "x5t#S512" (X.509 Certificate Thumbprint using
SHA-512) confirmation method member could be defined by registering
it in the the IANA "JWT Confirmation Methods" registry
[IANA.JWT.Claims] for JWT "cnf" member values established by
[RFC7800].
3.2. Confirmation Method for Token Introspection
OAuth 2.0 Token Introspection [RFC7662] defines a method for a
protected resource to query an authorization server about the active
state of an access token as well as to determine meta-information
about the token.
For a mutual TLS client certificate bound access token, the hash of
the certificate to which the token is bound is conveyed to the
protected resource as meta-information in a token introspection
response. The hash is conveyed using the same structure as the
certificate SHA-256 thumbprint confirmation method, described in
Section 3.1, as a top-level member of the introspection response
JSON. The protected resource compares that certificate hash to a
hash of the client certificate used for mutual TLS authentication and
rejects the request, if they do not match.
Proof-of-Possession Key Semantics for JSON Web Tokens [RFC7800]
defined the "cnf" (confirmation) claim, which enables confirmation
key information to be carried in a JWT. However, the same proof-of-
possession semantics are also useful for introspected access tokens
whereby the protected resource obtains the confirmation key data as
meta-information of a token introspection response and uses that
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information in verifying proof-of-possession. Therefore this
specification defines and registers proof-of-possession semantics for
OAuth 2.0 Token Introspection [RFC7662] using the "cnf" structure.
When included as a top-level member of an OAuth token introspection
response, "cnf" has the same semantics and format as the claim of the
same name defined in [RFC7800]. While this specification only
explicitly uses the "x5t#S256" confirmation method member, it needed
to define and register the higher level "cnf" structure as an
introspection response member in order to define and use the more
specific certificate thumbprint confirmation method.
The following is an example of an introspection response for an
active token with an "x5t#S256" certificate thumbprint confirmation
method.
HTTP/1.1 200 OK
Content-Type: application/json
{
"active": true,
"iss": "https://server.example.com",
"sub": "ty.webb@example.com",
"exp": 1493726400,
"nbf": 1493722800,
"cnf":{
"x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
}
}
Figure 2: Example Introspection Response for a Certificate Bound
Access Token
3.3. Authorization Server Metadata
This document introduces the following new authorization server
metadata parameter to signal the server's capability to issue
certificate bound access tokens:
tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value indicating server support for mutual TLS
client certificate bound access tokens. If omitted, the default
value is "false".
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Internet-Draft OAuth Mutual TLS May 20183.4. Client Registration Metadata
The following new client metadata parameter is introduced to convey
the client's intention to use certificate bound access tokens:
tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value used to indicate the client's intention
to use mutual TLS client certificate bound access tokens. If
omitted, the default value is "false".
4. Implementation Considerations4.1. Authorization Server
The authorization server needs to set up its TLS configuration
appropriately for the binding methods it supports.
If the authorization server wants to support mutual TLS client
authentication and other client authentication methods in parallel,
it should make mutual TLS optional.
If the authorization server supports the Self-Signed Certificate
method, it should configure the TLS stack in a way that it does not
verify whether the certificate presented by the client during the
handshake is signed by a trusted CA certificate.
The authorization server may also consider hosting the token
endpoint, and other endpoints requiring client authentication, on a
separate host name or port in order to prevent unintended impact on
the TLS behavior of its other endpoints, e.g. the authorization
endpoint.
4.2. Resource Server
Since the resource server relies on the authorization server to
perform client authentication, there is no need for the resource
server to validate the trust chain of the client's certificate in any
of the methods defined in this document. Mutual TLS is used only as
a proof-of-possession mechanism during protected resource access.
The resource server should therefore configure the TLS stack in a way
that it does not verify whether the certificate presented by the
client during the handshake is signed by a trusted CA certificate.
4.3. Certificate Bound Access Tokens Without Client Authentication
Mutual TLS OAuth client authentication and mutual TLS client
certificate bound access tokens can be used independently of each
other. Use of certificate bound access tokens without mutual TLS
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OAuth client authentication, for example, is possible in support of
binding access tokens to a TLS client certificate for public clients
or clients utilizing other methods of authentication to the
authorization server. The authorization server would configure the
TLS stack in the same manner as for the Self-Signed Certificate
method such that it does not verify that the certificate presented by
the client during the handshake is signed by a trusted CA.
Individual instances of a client would create a self-signed
certificate for mutual TLS with both the authorization server and
resource server. The authorization server would not use the mutual
TLS certificate to authenticate the client at the OAuth layer but
would bind the issued access token to that certificate, which the
client has proven possession of the corresponding private key. The
access token is then bound to the certificate and can only be used by
the client possessing the certificate and corresponding private key
and utilizing them to negotiate mutual TLS on connections to the
resource server.
4.4. Certificate Bound Access Tokens
As described in Section 3, an access token is bound to a specific
client certificate, which means that the same certificate must be
used for mutual TLS on protected resource access. It also implies
that access tokens are invalidated when a client updates the
certificate, which can be handled similar to expired access tokens
where the client requests a new access token (typically with a
refresh token) and retries the protected resource request.
4.5. Implicit Grant Unsupported
This document describes binding an access token to the client
certificate presented on the TLS connection from the client to the
authorization server's token endpoint, however, such binding of
access tokens issued directly from the authorization endpoint via the
implicit grant flow is explicitly out of scope. End users interact
directly with the authorization endpoint using a web browser and the
use of client certificates in user's browsers bring operational and
usability issues, which make it undesirable to support certificate
bound access tokens issued in the implicit grant flow.
Implementations wanting to employ certificate bound access tokens
should utilize grant types that involve the client making an access
token request directly to the token endpoint (e.g. the authorization
code and refresh token grant types).
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Internet-Draft OAuth Mutual TLS May 20184.6. TLS Termination
An authorization server or resource server MAY choose to terminate
TLS connections at a load balancer, reverse proxy, or other network
intermediary. How the client certificate metadata is securely
communicated between the intermediary and the application server in
this case is out of scope of this specification.
5. Security Considerations5.1. TLS Versions and Best Practices
TLS 1.2 [RFC5246] is cited in this document because, at the time of
writing, it is the latest version that is widely deployed. However,
this document is applicable with other TLS versions supporting
certificate-based client authentication. Implementation security
considerations for TLS, including version recommendations, can be
found in Recommendations for Secure Use of Transport Layer Security
(TLS) and Datagram Transport Layer Security (DTLS) [BCP195].
5.2. X.509 Certificate Spoofing
If the PKI method of client authentication is used, an attacker could
try to impersonate a client using a certificate with the same subject
DN but issued by a different CA, which the authorization server
trusts. To cope with that threat, the authorization server should
only accept as trust anchors a limited number of CAs whose
certificate issuance policy meets its security requirements. There
is an assumption then that the client and server agree on the set of
trust anchors that the server uses to create and validate the
certificate chain. Without this assumption the use of a Subject DN
to identify the client certificate would open the server up to
certificate spoofing attacks.
5.3. X.509 Certificate Parsing and Validation Complexity
Parsing and validation of X.509 certificates and certificate chains
is complex and implementation mistakes have previously exposed
security vulnerabilities. Complexities of validation include (but
are not limited to) [X509Pitfalls] [DangerousCode] [RFC5280]:
o checking of Basic Constraints, basic and extended Key Usage
constraints, validity periods, and critical extensions;
o handling of null-terminator bytes and non-canonical string
representations in subject names;
o handling of wildcard patterns in subject names;
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[X509Pitfalls]
Wong, D., "Common x509 certificate validation/creation
pitfalls", September 2016,
<https://www.cryptologie.net/article/374/common-x509-certificate-validationcreation-pitfalls>.
Appendix A. Relationship to Token Binding
OAuth 2.0 Token Binding [I-D.ietf-oauth-token-binding] enables the
application of Token Binding to the various artifacts and tokens
employed throughout OAuth. That includes binding of an access token
to a Token Binding key, which bears some similarities in motivation
and design to the mutual TLS client certificate bound access tokens
defined in this document. Both documents define what is often called
a proof-of-possession security mechanism for access tokens, whereby a
client must demonstrate possession of cryptographic keying material
when accessing a protected resource. The details differ somewhat
between the two documents but both have the authorization server bind
the access token that it issues to an asymmetric key pair held by the
client. The client then proves possession of the private key from
that pair with respect to the TLS connection over which the protected
resource is accessed.
Token Binding uses bare keys that are generated on the client, which
avoids many of the difficulties of creating, distributing, and
managing certificates used in this specification. However, at the
time of writing, Token Binding is fairly new and there is relatively
little support for it in available application development platforms
and tooling. Until better support for the underlying core Token
Binding specifications exists, practical implementations of OAuth 2.0
Token Binding are infeasible. Mutual TLS, on the other hand, has
been around for some time and enjoys widespread support in web
servers and development platforms. As a consequence, OAuth 2.0
Mutual TLS Client Authentication and Certificate Bound Access Tokens
can be built and deployed now using existing platforms and tools. In
the future, the two specifications are likely to be deployed in
parallel for solving similar problems in different environments.
Authorization servers may even support both specifications
simultaneously using different proof-of-possession mechanisms for
tokens issued to different clients.
Appendix B. Acknowledgements
Scott "not Tomlinson" Tomilson and Matt Peterson were involved in
design and development work on a mutual TLS OAuth client
authentication implementation, which predates this document.
Experience and learning from that work informed some of the content
of this document.
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Additionally, the authors would like to thank the following people
for their input and contributions to the specification: Sergey
Beryozkin, Vladimir Dzhuvinov, Samuel Erdtman, Leif Johansson,
Michael Jones, Phil Hunt, Benjamin Kaduk, Takahiko Kawasaki, Sean
Leonard, Kepeng Li, Neil Madden, James Manger, Jim Manico, Nov
Matake, Sascha Preibisch, Justin Richer, Dave Tonge, and Hannes
Tschofenig.
Appendix C. Document(s) History
[[ to be removed by the RFC Editor before publication as an RFC ]]
draft-ietf-oauth-mtls-08
o Incorporate clarifications and editorial improvements from Justin
Richer's WGLC review
o Drop the use of the "sender constrained" terminology per WGLC
feedback from Neil Madden (including changing the metadata
parameters from mutual_tls_sender_constrained_access_tokens to
tls_client_certificate_bound_access_tokens)
o Add a new security considerations section on X.509 parsing and
validation per WGLC feedback from Neil Madden and Benjamin Kaduk
o Note that a server can terminate TLS at a load balancer, reverse
proxy, etc. but how the client certificate metadata is securely
communicated to the backend is out of scope per WGLC feedback
o Note that revocation checking is at the discretion of the AS per
WGLC feedback
o Editorial updates and clarifications
o Update draft-ietf-oauth-discovery reference to -10 and draft-ietf-oauth-token-binding to -06
o Add folks involved in WGLC feedback to the acknowledgements list
draft-ietf-oauth-mtls-07
o Update to use the boilerplate from RFC 8174draft-ietf-oauth-mtls-06
o Add an appendix section describing the relationship of this
document to OAuth Token Binding as requested during the the
Singapore meeting https://datatracker.ietf.org/doc/minutes-100-oauth/
o Add an explicit note that the implicit flow is not supported for
obtaining certificate bound access tokens as discussed at the
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o Add/incorporate text to the Security Considerations on Certificate
Spoofing as suggested https://mailarchive.ietf.org/arch/msg/oauth/V26070X-6OtbVSeUz_7W2k94vCo
o Changed the title to be more descriptive
o Move the Security Considerations section to before the IANA
Considerations
o Elaborated on certificate bound access tokens a bit more in the
Abstract
o Update draft-ietf-oauth-discovery reference to -08
draft-ietf-oauth-mtls-05
o Editorial fixes
draft-ietf-oauth-mtls-04
o Change the name of the 'Public Key method' to the more accurate
'Self-Signed Certificate method' and also change the associated
authentication method metadata value to
"self_signed_tls_client_auth".
o Removed the "tls_client_auth_root_dn" client metadata field as
discussed in https://mailarchive.ietf.org/arch/msg/oauth/swDV2y0be6o8czGKQi1eJV-g8qc
o Update draft-ietf-oauth-discovery reference to -07
o Clarify that MTLS client authentication isn't exclusive to the
token endpoint and can be used with other endpoints, e.g. RFC7009 revocation and 7662 introspection, that utilize client
authentication as discussed in
https://mailarchive.ietf.org/arch/msg/oauth/bZ6mft0G7D3ccebhOxnEYUv4puI
o Reorganize the document somewhat in an attempt to more clearly
make a distinction between mTLS client authentication and
certificate bound access tokens as well as a more clear
delineation between the two (PKI/Public key) methods for client
authentication
o Editorial fixes and clarifications
draft-ietf-oauth-mtls-03
o Introduced metadata and client registration parameter to publish
and request support for mutual TLS sender constrained access
tokens
o Added description of two methods of binding the cert and client,
PKI and Public Key.
o Indicated that the "tls_client_auth" authentication method is for
the PKI method and introduced "pub_key_tls_client_auth" for the
Public Key method
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o Added implementation considerations, mainly regarding TLS stack
configuration and trust chain validation, as well as how to to do
binding of access tokens to a TLS client certificate for public
clients, and considerations around certificate bound access tokens
o Added new section to security considerations on cert spoofing
o Add text suggesting that a new cnf member be defined in the
future, if hash function(s) other than SHA-256 need to be used for
certificate thumbprints
draft-ietf-oauth-mtls-02
o Fixed editorial issue https://mailarchive.ietf.org/arch/msg/oauth/U46UMEh8XIOQnvXY9pHFq1MKPns
o Changed the title (hopefully "Mutual TLS Profile for OAuth 2.0" is
better than "Mutual TLS Profiles for OAuth Clients").
draft-ietf-oauth-mtls-01
o Added more explicit details of using RFC 7662 token introspection
with mutual TLS sender constrained access tokens.
o Added an IANA OAuth Token Introspection Response Registration
request for "cnf".
o Specify that tls_client_auth_subject_dn and
tls_client_auth_root_dn are RFC 4514 String Representation of
Distinguished Names.
o Changed tls_client_auth_issuer_dn to tls_client_auth_root_dn.
o Changed the text in the Section 3 to not be specific about using a
hash of the cert.
o Changed the abbreviated title to 'OAuth Mutual TLS' (previously
was the acronym MTLSPOC).
draft-ietf-oauth-mtls-00
o Created the initial working group version from draft-campbell-oauth-mtlsdraft-campbell-oauth-mtls-01
o Fix some typos.
o Add to the acknowledgements list.
draft-campbell-oauth-mtls-00
o Add a Mutual TLS sender constrained protected resource access
method and a x5t#S256 cnf method for JWT access tokens (concepts
taken in part from draft-sakimura-oauth-jpop-04).
o Fixed "token_endpoint_auth_methods_supported" to
"token_endpoint_auth_method" for client metadata.
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